The invention relates to the field of concrete, more specifically conductive concrete.
Conventional concrete, consisting of hydrated Portland cement with silica sand as fine aggregate and limestone, stone or other coarse aggregate, is a good electrical insulator. The electrical resistivity of conventional concrete usually ranges from about 6.54 to 11.4xc3x97105 xcexa9cm for dried concrete and about 2.5 to 4.5xc3x97103 xcexa9cm for moist concrete (conductors such as metals have resistivities in the order of 10xe2x88x925 xcexa9cm).
For some end uses, it is desirable to have concrete that is a conductor rather than an insulator. Conductive concrete is useful as electromagnetic shielding. It is often required in the design and construction of facilities and equipment to protect electrical systems or electronic components from the effects of unwanted electromagnetic energy. Other applications are anti-static flooring in the electronic instrumentation industry and in hospitals; and cathodic protection of steel reinforcement in concrete structures. Another use for conductive concrete is for heating purposes. Slabs of conductive concrete can be used to melt snow or ice (for example on an airport runway), or for radiant indoor heating.
Conductive concrete compositions have been described in the technical and patent literature. Banthia et al, Cem. Concr. Res., 22(5), 804-814 (1992), studied the electrical resistivity of carbon fiber- and steel microfiber-reinforced cements. The content of conductive fiber ranged from 1 to 5% by volume. The resistivity at 28 days of hydration ranged from 78 xcexa9cm to 31.92 xcexa9cm.
Kojima et al, CAJ Proceedings of Cement and Concrete, The Cement Association of Japan, No. 43, 560-565 (1989), prepared a conductive carbon fiber/cement composite by laminating six sheets of carbon fibre paper impregnated with Portland cement paste. The product was 3 mm thick and had a resistivity value of 0.7 xcexa9cm. The material was highly effective in electromagnetic shielding, however, the raw materials are expensive and the concrete product is not suitable for load-bearing applications.
Chiou et al, Composites, 20(4), 379-381 (1989), reported work on carbon fiber reinforced cement for electromagnetic shielding, with results similar to those of Banthia et al. Shintani et al, in U.S. Pat. No. 5,422,174 describe a conductive concrete for electromagnetic wave shielding; the concrete, which relies on carbon fibres as the conductive element, is spread on a panel of regular concrete or plasterboard in order to have sufficient mechanical strength for building purposes. McCormack, in U.S. Pat. No. 5,346,547 describes conductive concrete containing magnetized steel fibres. The concrete compositions described in the above-mentioned literature are useful for specific purposes; however, they all represent a compromise between high mechanical strength and conductivity.
Xie et al, in U.S. Pat. No. 5,447,564 describe a conductive concrete composition which exhibits both low resistivity (as low as 0.46 xcexa9cm) and high mechanical strength (over 30 Mpa).
There remains a need for conductive concrete compositions combining good mechanical strength and high electrical conductivity, suitable for commercial large-scale application.
In a first aspect, the invention provides a method for making conductive concrete, the method comprising the steps of:
(A) mixing porous conductive carbonaceous particles with water, thus forming pre-saturated carbonaceous particles;
(B) adding to the pre-saturated carbonaceous particles a cement binder, and a superplasticiser together with additional water, if required, to form a fresh concrete mixture;
(C) moulding and compacting the fresh concrete mixture; and
(D) curing the compacted fresh concrete mixture.
In a second aspect, the invention provides a method for making conductive concrete, the method comprising the steps of:
(A) mixing, in any order, conductive carbonaceous particles, a cement binder, superplasticiser and water, to form a fresh concrete mixture;
(B) moulding and compacting the fresh concrete mixture; and
(C) air-curing the compacted fresh concrete mixture.
The method is particularly suited to large-scale commercial production of conductive concrete, using an industrial scale mixer (for example, having a capacity of about 0.08 m3 to about 12 m3). The method is suitable for conductive concrete made in drum mixers, pan mixers and high shear mixers.